His plan to attain the first-mentioned object was simple, and promised to be successful.  Jackson was to march around by way of “Williamsport and Martinsburg,” and thus approach from the south.  A force was meanwhile to seize upon and occupy the Maryland Heights, a lofty spot of the mountain across the Potomac, north of the Ferry.  In like manner, another body of troops was to cross the Potomac, east of the Blue Ridge, and occupy the Loudon Heights, looking down upon Harper’s Ferry from the east.  By this arrangement the retreat of the enemy would be completely cut off in every direction.  Harper’s Ferry must be captured, and, having effected that result, the whole Confederate force, detached for the purpose, was to follow the main body of this army in the direction of Hagerstown, to take part in the proposed invasion of Pennsylvania.

This excellent plan failed, as will be seen, from no fault of the great soldier who devised it, but in consequence of unforeseen obstacles, and especially of one of those singular incidents which occasionally reverse the best-laid schemes and abruptly turn aside the currents of history.

Jackson and the commanders cooeperating with him moved on September 10th.  General Lee then with his main body crossed the South Mountain, taking the direction of Hagerstown.  Meanwhile, General McClellan had advanced cautiously and slowly, withheld by incessant dispatches from Washington, warning him not to move in such a manner as to expose that city to danger.  Such danger existed only in the imaginations of the authorities, as the army in advancing extended its front from the Potomac to the Baltimore and Ohio Railroad.  General McClellan, nevertheless, moved with very great precaution, feeling his way, step by step, like a man in the dark, when on reaching Frederick City, which the Confederates had just evacuated, good fortune suddenly came to his assistance.  This good fortune was the discovery of a copy of General Lee’s orders of march for the army, in which his whole plan was revealed.  General McClellan had therein the unmistakable evidence of his opponent’s intentions, and from that moment his advance was as rapid as before it had been deliberate.

The result of this fortunate discovery was speedily seen.  General Lee, while moving steadily toward Hagerstown, was suddenly compelled to turn his attention to the mountain-passes in his rear.  It had not been the intention of Lee to oppose the passage of the enemy through the South Mountain, as he desired to draw General McClellan as far as possible from his base, but the delay in the fall of Harper’s Ferry now made this necessary.  It was essential to defend the mountain-defiles in order to insure the safety of the Confederate troops at Harper’s Ferry; and Lee accordingly directed General D.H.  Hill to oppose the passage of the enemy at Boonsboro Gap, and Longstreet was sent from Hagerstown to support him.